1. Field of the Invention
The present invention relates to a nitride-based semiconductor device having a compound semiconductor layer made of a group III-V nitride-based semiconductor (hereinafter referred to as nitride-based semiconductor) such as GaN (gallium nitride), AlN (aluminum nitride), InN (indium nitride), BN (boron nitride) or TlN (thallium nitride) or mixed crystal thereof, and a manufacturing method thereof.
2. Description of the Background Art
In recent years, researches have been carried out into GaN-based nitride-based semiconductor devices. In the manufacture of the GaN-based semiconductor device, a substrate of sapphire in the hexagonal system the same as that of GaN is used, since there is no substrate made of GaN. A GaN-based semiconductor layer is grown on the sapphire substrate.
Herein, the lattice constant (a axis) of GaN crystal is 3.19 Å, while the lattice constant (a axis) of sapphire crystal is 4.8 Å. Thus, GaN and sapphire have significantly different lattice constants, and therefore there is strain at a GaN layer grown on the sapphire substrate. The strain causes a large number of lattice defects in the GaN layer. The defect density of the GaN layer grown on the sapphire substrate is about in the range from 1011 to 1012 cm−2.
The GaN lattice defects described above are transmitted into a semiconductor layer grown on the GaN layer, and therefore a semiconductor device manufactured using a sapphire substrate has a large number of lattice defects. The lattice defects cause current leakage and impurity diffusion in the semiconductor device, and thus adversely affect the characteristics of the semiconductor device.
In a semiconductor laser device with a large number of lattice defects, for example, much leakage current is generated. Therefore, the operation current of the semiconductor laser device increases. As a result, the semiconductor laser device suffers from considerable deterioration, which shortens the useful life of the device.
Therefore, in order to reduce the lattice defects as described above, the following conventional methods have been employed.
According to one method, a GaN-based semiconductor layer is grown through an InGaN layer on a GaN layer grown on a sapphire substrate and having many lattice defects.
As shown in FIG. 11 at (a), the lattice constant (a axis) of InN crystal is 3.55 Å which is closer to the lattice constant of sapphire than GaN. The method takes advantage of the relation, and the GaN-based semiconductor layer is grown through the InGaN layer having the lattice constant closer to sapphire to reduce the lattice defects. According to the method, however, the lattice defects cannot be reduced sufficiently.
Meanwhile, according to another method, an InGaN/GaN multi-layer film including InGaN films and GaN films alternately layered on one another is formed on a GaN layer grown on a sapphire substrate and having many lattice defects and a GaN-based semiconductor layer is grown through the InGaN/GaN multi-layer film.
In this case, since the lattice constant of InN is closer to that of sapphire than GaN, as shown in FIG. 11 at (b), the average lattice constant of the InGaN/GaN multi-layer film is closer to the lattice constant of sapphire as compared to the case of using only a GaN layer. Also in this case, in the multi-layer structure including a GaN film and an InGaN film, the lattice constant is closer to that of GaN as compared to the case of using only an InGaN layer. The method takes advantage of the relation, and a GaN-based semiconductor layer is grown through an InGaN/GaN multi-layer film to reduce the lattice defects. According to the method, however, the lattice defects cannot be reduced sufficiently.
According to yet another method, an AlGaN/GaN multi-layer film including AlGaN films and GaN films alternately layered on one another is formed on a GaN layer grown on a sapphire substrate and having many lattice defects, and a GaN-based semiconductor layer is grown through the AlGaN/GaN multi-layer film.
In this case, the lattice constant (a axis) of AlN crystal is 3.11 Å and therefore as shown in FIG. 11 at (c), the average lattice constant of the AlGaN/GaN multi-layer film is smaller than the lattice constant of GaN. According to the method, strain causing lattice defects is concentrated in the AlGaN/GaN multi-layer film to reduce the lattice defects. According to the method, however, the lattice defects cannot be reduced sufficiently.
Meanwhile, according to another method disclosed by Japanese Patent Laid-Open No. 8-56015, an AlGaN/InGaN multi-layer film including AlGaN films and InGaN films alternately layered on one another is formed on a GaN layer grown on a sapphire substrate and having many lattice defects, and a GaN-based semiconductor layer is grown through the AlGaN/InGaN multi-layer film.
According to the method, an AlGaN film is grown on a GaN layer having many lattice defects at a substrate temperature of 800° C., and then an InGaN film is grown. Herein, a pair of an AlGaN film and an InGaN film is referred to as one cycle, and a series of forty such cycles altogether are formed. The AlGaN/InGaN multi-layer film thus formed is used to reduce lattice defects extended from the GaN layer.
In the AlGaN/InGaN multi-layer film as described above, however, the AlGaN film is grown first on the GaN layer at a substrate temperature of 800° C., and therefore the crystallinity of the AlGaN film formed first in the AlGaN/InGaN multi-layer film is poor. In the AlGaN/InGaN multi-layer film including a plurality of AlGaN films and InGaN films layered on one another, the crystallinity of the film grown first, in other words the crystallinity of the AlGaN film affects the crystallinity of films subsequently formed thereupon. Therefore, the poor crystallinity of the AlGaN film formed first keeps the AlGaN/InGaN multi-layer film from having good crystallinity, and the lattice defects cannot be reduced sufficiently.
Also according to the method described above, the forty cycles of AlGaN and InGaN film pairs are formed, and the AlGaN/InGaN multi-layer film having so many layers of AlGaN films cannot have good crystallinity. Therefore, the lattice defects cannot be reduced sufficiently.
As described above, according to the method disclosed by Japanese Patent Laid-Open No. 8-56015, the characteristics of the semiconductor device can hardly be improved.
Meanwhile, selective lateral growth using a selective growth mask is known as a method of reducing lattice defects. According to the method, a stripe-shaped selective growth mask is formed on a GaN layer grown on a sapphire substrate and having many lattice defects. Then, according to HVPE (Halide Vapor Phase Epitaxy) process, GaN is re-grown on the GaN layer and the selective growth mask. Thus, the lattice defects in the re-grown GaN layer can be reduced to a level of about 6×107 cm−2 in the defect density.
The method using such a selective growth mask is most widely employed for reducing lattice defects in the semiconductor device. At present, nitride-based semiconductor laser devices having a long useful life are provided only by this method.
According to the method, however, a wafer must be once taken out from a crystal growth system to form a selective growth mask thereon, and then the wafer must be returned into the crystal growth system after the selective growth mask is formed. This complicates the process of manufacturing the semiconductor device, which pushes up the manufacturing cost.